1. Field of the Invention
This invention relates to operational amplifiers, and more specifically to a unique operational amplifier having three input terminals, allowing for cancellation of the inherent offset voltage of the operational amplifier while providing an inverting and a noninverting input lead for receiving two non-sampled input signals.
2. Description of the Prior Art
Prior art operational amplifiers are well-known. In the manufacture of operational amplifiers in the form of integrated circuits, fabrication tolerances result in component mismatches, thus providing each operational amplifier with its own unique inherent offset voltage V.sub.off. This offset voltage is defined as the output voltage of the operational amplifier when the amplifier is in the unity gain mode (inverting input lead and output lead connected) and its noninverting input lead grounded. Because each operational amplifier has its own unique offset voltage, each circuit utilizing such an operational amplifier must compensate in a unique manner for the inherent offset voltage associated with that specific operational amplifier.
Several prior art methods of providing compensation for the inherent offset voltage of an operational amplifier are known. However, such methods require that the offset voltage be stored on an offset capacitor connected to the inverting input lead of the operational amplifier, thus preventing the inverting input lead from being utilized as an input lead for receiving a continuous, non-sampled input signal. Such prior art methods allow the inverting input lead to be utilized for receiving an input signal only if the input signal is sampled and stored on an input capacitor, and the input capacitor is then connected to the inverting input lead of the operational amplifier. In this manner, the offset voltage stored on the offset capacitor is prevented from being discharged. Such a method and structure are disclosed in my copending U.S. patent application Ser. No. 185,356 filed Sept. 8, 1980 and assigned to American Microsystems, Inc., the assignee of this application, which is hereby incorporated by reference.